1. Field of the Invention
The present invention relates to a ceramic support for supporting a catalyst component for cleaning an exhaust gas from an internal combustion engine of an automobile, etc., a catalyst-ceramic body using the ceramic support, and processes for producing these.
2. Description of the Related Art
A widely used conventional catalyst support for a catalyst component used to clean an exhaust gas is a honeycomb structure of cordierite, which is excellent in thermal shock resistance, the surface of which is coated with γ-alumina. This is because a cordierite honeycomb structure has a relatively small specific surface area such that a sufficient amount of a catalyst component cannot be supported thereon. Therefore, γ-alumina having a large specific surface area is coated on the surface of a cordierite honeycomb structure, to support a catalyst component such as Pt thereon.
Because recent changes in the exhaust gas regulation, a quick activation of a catalyst is required to reduce hydrocarbons from an exhaust gas generated immediately after the starting of an engine. To satisfy to this requirement, a reduction of the thermal capacity of a catalyst support, in order to increase the temperature elevating rate of a catalyst to an activated state has been considered. Reduction of the thermal capacity of a catalyst support can be made by decreasing the thickness of cell walls of a cordierite honeycomb structure. However, even if the thickness of cell walls of a cordierite honeycomb structure is reduced, to have a small thermal capacity, the amount of a coating of γ-alumina does not decrease, resulting in lowering the effect of reducing the thermal capacity of the honeycomb structure as a whole.
In reducing the thickness of cell walls of a cordierite honeycomb structure, a reduction in pressure loss can be expected but this effect is also reduced by a coating of γ-alumina. Further, although a cordierite honeycomb structure itself has a small thermal expansion coefficient, about 0.5×10−6/° C., the thermal expansion coefficient of a honeycomb structure with a coating of γ-alumina increases to about 1.5×10−6/° C., by which the thermal shock resistance of the honeycomb structure significantly decreases.
In order to solve the above problems, an increase in the specific surface area of a cordierite honeycomb structure to eliminate the necessity of a coating of γ-alumina has been considered. For example, Japanese Examined Patent Publication (Kokoku) No. 5-50338 discloses a method of carrying out acid and heat treatments of a cordierite honeycomb structure to increase the specific surface area thereof. Although it is mentioned that this method allows increase in the specific surface area from 1 m2/g to about 30 m2/g, this method disadvantageously decreases the strength of the cordierite honeycomb structure. Since the acid treatment selectively dissolves MgO and Al2O3 from the cordierite to increase the surface area, the crystal lattice of cordierite itself is destroyed and the strength of the cordierite honeycomb structure decreases.
A honeycomb structure should have a compressive strength in the direction of the flow channel as a high as 10 MPa or more to be installed in an exhaust gas cleaning catalyst converter. In the method disclosed in JPP'338, a compressive strength, in the direction of the flow channel, of 10 MPa or more cannot be attained if the treated cordierite honeycomb structure has a cell wall thickness of less than 150 μm at a cell density of 400 cpsi (cells per square inch). However, a cordierite honeycomb structure having a cell wall thickness of 150 μm at a cell density of 400 cpsi has a thermal capacity equivalent to that of a cordierite honeycomb structure having a cell wall thickness of 120 μm at a cell density of 400 cpsi and coated with γ-alumina at 180 g/l. Therefore, the above method in JPP'338 does not provide an improved effect in reducing the thermal capacity of a catalyst support, considering the fact that a cordierite honeycomb structure having a cell wall thickness of 85 μm at a cell density of 400 cpsi can be produced at present.
It is therefore required that a cordierite honeycomb structure for cleaning an exhaust gas has a compressive strength in the direction of the flow channel of 10 MPa or more with a cell wall thickness of 120 μm or less at a cell density of 400 cpsi and can support a required amount of a catalyst component without a coating of γ-alumina.
Thus, the object of the present invention is to make a ceramic support itself which can support a required amount of a catalyst component, by which the necessity of coating with γ-alumina is eliminated, and to thereby provide a ceramic support to be used to support a catalyst component for cleaning an exhaust gas from an internal combustion engine of an automobile, etc., which has a reduced thermal capacity and pressure loss as well as a lower thermal expansion coefficient. Also, the object of the present invention is to provide a catalyst-ceramic body and processes for producing the ceramic support and the catalyst-ceramic body.